Interaction of serotonin- and dopamine-related neurotoxins with "serotonin binding proteins" in bovine frontal cortex

Binding of [3H]serotonin and [3H]dopamine to serotonin-binding proteins (SBP) from soluble extracts of bovine frontal cortex is increased by Fe2+. This group recently attributed this effect of Fe2+ to its ability to enhance the oxidation of [3H]serotonin and [3H]dopamine in the presence of dissolved molecular oxygen, and to the ability of the formed oxidation products to bind covalently to cysteine residues of SBP. In this study it is shown that the binding of both ligands is potently inhibited by dopamine as well as by several catecholamine-and serotonin-related neurotoxins: adrenochrome, 5,6-dihydroxytryptamine, 5,7-dihydroxytryptamine, 6-hydroxydopamine and 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline. In contrast, serotonin can only potently inhibit part (36%) of the [3H]dopamine binding, while 1,2,3,4-tetrahydroisoquinoline is only a weak competitor for both ligands. Potent inhibition by the toxins is associated with the presence of electrophilic centres at the aromatic ring, either of the products themselves (adrenochrome) or of their oxidation products (all other competitors). These findings suggest that "SBP" represent an important target for the Fe(2+)-mediated binding of [3H]-serotonin, [3H]dopamine and related neurotoxins.

[3H]Clozapine is not a suitable radioligand for the labelling of D4 dopamine receptors in postmortem human brain

Clozapine displays nanomolar affinity for cloned human D4-type dopamine receptors expressed in tissue culture cells. Therefore, [3H]clozapine has been introduced as a radioligand for the labelling of the D4 dopamine receptors. We found that, in membranes of postmortem human striatum, amygdala, frontal cortex and substantia nigra, neither dopamine nor the dopamine agonist apomorphine displaced the binding of [3H]clozapine (5-20 nM). [3H]Clozapine competition curves with clozapine and loxapine revealed the presence of two binding sites. The high-affinity site was displaced by atropine and pirenzepine with nanomolar affinity. The low-affinity site did not correspond to a serotonin, adrenergic, gamma-amino butyric acid, N-methyl-D-aspartate, benzodiazepine, histamine, sigma, imidazoline receptor-binding site, or catecholamine uptake site. Our results suggest that [3H]clozapine in postmortem human brain binds to M1-type muscarinic cholinergic receptors and to a low-affinity binding site but is not a suitable radioligand for investigating the D4 dopamine receptors.

Insulin-like growth factor-I receptors in human brain and pituitary gland: an autoradiographic study

Insulin-like growth factor (IGF)-I receptors were studied in adult human postmortem brain and pituitary gland using quantitative autoradiography with human recombinant [125I]IGF-I. The highest densities were found in the choroid plexus, pituitary gland--where IGF-I receptors were mainly concentrated in the anterior lobe, pineal gland, glomerular layer of the olfactory bulb, and the molecular layer of the cerebellar cortex. Moderate densities were present in cerebral cortex, caudate nucleus, putamen, accumbens, the CA1, CA2, CA3 fields and dentate gyrus of the hippocampus, the dentate nucleus of the cerebellum, amygdala, thalamus, pontine nuclei, and substantia nigra. All other brain areas, including white matter, contained low densities of IGF-I receptors. The finding that several well-defined brain structures are enriched with IGF-I receptors suggests a neurotrophic/survival or neuromodulatory role of insulin-like growth factors on specific neuronal systems. IGF-I receptors observed in the white matter may be associated with oligodendrocytes.

Autoradiographic distribution of D3-type dopamine receptors in human brain using [3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin

The regional distribution of D3 dopamine receptors was studied in human brain by quantitative autoradiography with [3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin ([3H]7-OH-DPAT). The highest densities were found in the ventral striatum/nucleus accumbens, followed by the remainder of the neostriatum, cerebral cortex, and cerebellar cortex. Moderate amounts were found in the substantia nigra. Low densities of D3 dopamine receptors were visualized in pituitary gland (posterior lobe > anterior lobe), amygdala, and hippocampus. The globus pallidus and thalamus contained lower densities. The distribution pattern is much more widespread as detected by in situ hybridisation histochemistry for D3 mRNA in human and in rat brain. Our data confirm a predilection of D3 receptors in brain areas involved in cognitive and emotional functions. The presence of D3 receptors in non-limbic parts of the neostriatum and substantia nigra suggests that they also play a role in the dopaminergic control of motor processes. Its precise function in cerebellum and pituitary gland is at present uncertain.

Imidazoline receptors, non-adrenergic idazoxan binding sites and alpha 2-adrenoceptors in the human central nervous system

Both [3H]clonidine and [3H]idazoxan bind to alpha 2 adrenoceptors. The former also labels imidazoline receptors, and the latter non-adrenergic idazoxan binding sites. In order to investigate whether the imidazoline receptors and non-adrenergic idazoxan binding sites are identical, we compared the binding characteristics of [3H]clonidine and [3H]idazoxan to these sites by radioligand binding experiments on ultra-thin slices and homogenates of human striatum. A good correlation was found between the effect of different ions on the binding characteristics of [3H]clonidine and [3H]idazoxan, and the affinities of most competing drugs. However, clonidine and rilmenidine displayed a 100- and 10-fold lower affinity, respectively, for the idazoxan binding sites than for the imidazoline receptors. Autoradiography with [3H]clonidine showed that high densities of imidazoline receptors were present in the striatum, pallidum, gyrus dentatus of the hippocampus, amygdala, and substantia nigra. Moderate densities were found throughout the cerebral cortex, thalamus and several brainstem nuclei including the nucleus olivarius inferior. Low densities were seen in the cerebellum, spinal cord and pituitary gland. As for the non-adrenergic sites labelled by [3H]idazoxan, the imidazoline receptors can be found in all major brain areas examined. However, there are some striking differences between the concentrations of imidazoline receptors and non-adrenergic idazoxan binding sites in certain brain regions. To reconcile distribution and pharmacologic data, we propose that imidazoline receptors and non-adrenergic idazoxan binding sites represent different proteins or protein complexes and that at least in the nucleus reticularis lateralis and the striatum, imidazoline receptors and non-adrenergic idazoxan binding sites may be physically associated. The regional distribution of alpha 2 adrenoceptors within the human CNS was determined by quantitative autoradiography with [3H]RX821002. The highest densities of alpha 2 adrenoceptors were found in the cerebral and cerebellar cortex, and certain regions in the medulla oblongata (floor of the IV ventricle, reticular formation, hypoglossal nucleus and nucleus olivarius inferior). No alpha 2 adrenoceptors were detected in the pituitary gland. There exists no relationship between the distribution pattern of imidazoline receptors and alpha 2 adrenoceptors, indicating that these binding sites are independent from each other.

Mivazerol, a novel compound with high specificity for alpha 2 adrenergic receptors: binding studies on different human and rat membrane preparations

Mivazerol, 3-[1(H-imidazol-4-yl)methyl]-2-hydroxybenzamide hydrochloride, a new potential anti-ischemic drug designed by UCB S.A. Pharma Sector, has been studied in binding experiments on adrenergic, dopaminergic, serotoninergic, muscarinic and idazoxan binding sites. Our results indicate that this compound displays high affinity and marked specificity for alpha 2 adrenoceptors. Mivazerol displaced the binding of the alpha 2 adrenoceptor antagonist [3H]RX 821002 to the alpha 2A adrenoceptors in human frontal cortex membranes with an apparent Ki value of 37 nM. The competition curve was shallow (nH = 0.55), suggesting that this compound acts as an alpha 2 adrenergic agonist. Mivazerol was also a potent competitor for [3H]RX 821002 binding to human platelet membranes (containing alpha 2A adrenoceptors) and rat kidney membranes (75% of the alpha 2 adrenoceptors of the alpha 2B subtype), indicating that this compound is not alpha 2 adrenoceptor subtype selective. Equilibrium dissociation constants for alpha 1 adrenoceptors (displacement of [3H]prazosin) and 5-HT1A receptors (displacement of [3H]rauwolscine) were respectively about 120 times (Ki = 4.4 microM) and 14 times (Ki = 530 nM) higher than that for the alpha 2 adrenoceptors. Equilibrium dissociation constants were approximately 1000 times higher for all other receptors tested in this study; namely beta 1 and beta 2 adrenoceptors, D1- and D2-dopamine receptors, M1-, M2- and M3-muscarinic receptors, 5-HT2 receptors and non-adrenergic idazoxan binding sites.

No functional atypical beta-adrenergic receptors in human omental adipocytes

Isoproterenol stimulates lipolysis in human omental adipocytes with an EC50 (concentration at which an agonist produces half-maximal stimulation) of 120 nM. CGP12177 (dl-4-3[(1,1-dimethylethyl) amino]-2-hydroxylpropoxy]1,3-dihydro-2H-benzimidazol-2-one hydrochloride), a potent beta 1-/beta 2- adrenergic receptor (AR) antagonist but being an agonist for atypical beta-AR, fails to stimulate lipolysis in these cells, even at a concentration as high as 0.1 mM. Since CGP12177 is a partial agonist, its failure to stimulate lipolysis may result from a poor stimulus-response coupling, so that it can not be excluded that atypical beta-AR are actually present and even functional in these cells. To evaluate this hypothesis, we estimated the potency of CGP12177 to inhibit the isoproterenol-stimulated lipolysis. This inhibition curve reflects a single class of sites and the IC50-value (concentration at which an antagonist produces half-maximal inhibition) of CGP12177 (3.8 nM) is in good agreement with what should be expected for beta 1-AR/beta 2-AR. Moreover, metoprolol and atenolol, two beta 1- AR- selective antagonists, shift the isoproterenol dose-response curve to the right with high potency as well. These potencies are similar to the ones found for beta 1-AR in the human heart but appreciably higher than those which should be expected for atypical beta-AR. The present study suggest that atypical beta-AR are not functional in human omental adipocytes.

Species and strain-related differences in the expression and functionality of beta-adrenoceptor subtypes in adipose tissue

The beta-adrenoceptor subtypes which trigger lipolysis in white adipocytes vary markedly between calf and rats, and even between different rat strains. In calf adipocytes, CGP12177, a potent antagonist for beta 1- and beta 2-adrenoceptors (i.e., "classical beta-adrenoceptors") and a partial agonist for atypical beta-adrenoceptors, did not stimulate lipolysis, but inhibited with high affinity (IC50 = 0.66 nM) the lipolytic response to 10 nM isoproterenol. In adipocytes from both Wistar rats and Sprague-Dawley OFA rats, CGP12177 stimulated lipolysis to almost the same extent as isoproterenol. Low concentrations of CGP12177 (3 nM) inhibited part of the lipolytic response to 10 nM isoproterenol in the Sprague-Dawley OFA rat adipocytes, but not in Wistar rats at all ages tested (2-4 weeks, 2-4 months, 24-26 months). Hence, functional beta-adrenoceptors are only classical in calf adipocytes, only atypical in Wistar rat adipocytes and both classical and atypical in Sprague-Dawley OFA rat adipocytes. Binding experiments were performed with 150 pM [125I]CYP. On calf adipocyte membranes, competition binding curves with CGP12177 displayed one high affinity binding site (IC50 = 4.7 nM), whereas the curves for CGP20712 (beta 1-selective antagonist) and ICI118551 (beta 2-selective antagonist) were biphasic. In agreement with the functional data, these results indicate that only beta 1- and beta 2-adrenoceptors are present in calf adipose tissue. For both rat strains, only half of the displaceable [125I]CYP binding sites displayed high affinity for CGP12177 (IC50 = 6.8 to 7.5 nM), and competition binding studies with CGP20712 and ICI118551 indicated that they represent beta 1- and beta 2-adrenoceptors. The remaining [125I]CYP binding sites possessed an about 50 times lower affinity for CGP12177 (IC50 = 260 to 345 nM). They are likely to represent atypical beta-adrenoceptors. It is concluded that the presence and the physiological relevance of beta-adrenoceptor subtypes in adipose tissue may not only be species-related, but also strain-related.

What intracranial tissues in humans contain sumatriptan-sensitive serotonin 5-HT1-type receptors?

We investigated the presence of sumatriptan-sensitive serotonin (5-HT)1 receptors in different human tissues by using a radioligand-binding technique with [3H]5-HT. Sumatriptan displaced [3H]5-HT from frontal cortical and striatal membranes in a biphasic manner, with a high-affinity site corresponding to binding to the 5-HT1D receptor. In blood platelet membranes, sumatriptan displaced [3H]5-HT with a 100-fold lower affinity. Sumatriptan failed to displace [3H]5-HT in membranes from large cerebral arteries, pial vessels, coronary arteries and dura mater. These findings suggest that either there are no sumatriptan-sensitive 5-HT1 receptors on intracranial blood vessels or they are so small in number that they cannot be detected by the radioligand-binding technique. Other mechanisms, possibly centrally mediated, may be responsible for the antimigraine action of sumatriptan.

Soluble serotonin and catecholamine binding proteins in the bovine adrenal medulla

The soluble serotonin-binding proteins (SBP) present in the adrenal medulla and in chromaffin cells, are very similar to those reported for the bovine brain and retina. Binding of [3H]serotonin and [3H]dopamine to these SBP is increased by Fe2+ but not by Fe3+. At an optimal concentration of Fe2+ (0.1 mM) these proteins behave as a single class of non-cooperative sites for [3H]serotonin (Bmax = 124 +/- 28 pmol/mg protein, KD = 0.51 +/- 0.13 microM) and [3H]dopamine (Bmax = 685 +/- 118 pmol/mg protein, KD = 0.46 +/- 0.06 microM). Binding of [3H]dopamine is also increased by Cu2+ and Mn2+, but to a lesser extent than by Fe2+. Catecholamines are good competitors for [3H]serotonin binding (Ki = 0.31 microM for dopamine, 0.6 microM for adrenaline and 0.9 microM for noradrenaline). The serotonin binding proteins from adrenal medulla elute in the void volume of a Sephacryl 100 HR gel filtration column, reflecting aggregation, and migrate mainly with an apparent molecular weight of 45 kDa in native polyacrylamide gel electrophoresis experiments. Subcellular localization studies and release experiments suggest that SBP are not present in chromaffin granules, but in the cytosol of purified chromaffin cells. The present data suggest that these proteins must have other functions than storing monoamines in synaptic vesicles.

Binding of serotonin and dopamine to 'serotonin binding proteins' in bovine frontal cortex: evidence for iron-induced oxidative mechanisms

Binding of [3H]serotonin and of [3H]dopamine to serotonin binding proteins (SBP) from soluble extracts of bovine frontal cortex is increased by Fe2+ but not by Fe3+. It was generally believed that Fe2+ first binds to sulfhydryl groups of SBP and that the monoamines form coordination bonds with the trapped iron. We report two series of findings that are incompatible with this mechanism. First, the binding of both radioligands is an irreversible process since it is not diminished when a large excess (1 mM) of serotonin or dopamine is added to a pre-equilibrated mixture of SBP, 0.1 mM Fe2+ and 0.2 microM radioligand. Once formed, binding is not impaired by chelating agents such as ethyleneglycoltetraacetic acid and desferal. Second, the Fe(2+)-stimulated binding is inhibited by reducing agents (sodium ascorbate, vitamin E, sodium metabisulfite) and by agents which deplete superoxide radicals (superoxide dismutase and hydrogen peroxide). Moreover, the effect of Fe2+ can be mimicked by oxidants (sodium periodate, potassium superoxide) and by the generation of superoxide radicals by the xanthine oxidase-catalysed oxidation of xanthine. To integrate these findings, we formulate the hypothesis that Fe2+ reacts with dissolved molecular oxygen to produce superoxide radicals, that these radicals oxidise [3H]serotonin and [3H]dopamine, and that the formed oxidation products bind covalently to cysteine residues of SBP. This alternative mechanism is also based on the ability of reagents which contain or modify sulfhydryl groups to decrease the binding and on the inability of hydroxyl radical scavengers (dimethyl sulfoxide, mannitol, ethanol and thiourea) to do so. Fe2+ is also able to irreversibly inactivate part of the binding sites on SBP (81% of the specific binding of [3H]serotonin, and 61% for [3H]dopamine). This Fe(2+)-mediated inactivation, as well as the covalent nature of the binding, preclude the interpretation of saturation and competition binding data in terms of reversible bimolecular interactions. Yet, such experiments indicate that, at the same concentration, [3H]dopamine binds to 2 to 3 times more sites than [3H]serotonin. Unlabelled dopamine acts also as a potent competitor at all the [3H]serotonin binding sites, whereas unlabelled serotonin only acts as a potent competitor at part (30%) of the [3H]dopamine binding sites. SBP were initially proposed to be involved in the storage, protection and/or transport of serotonin, and recently also of catecholamines. However, these potential functions of SBP can hardly be reconciled with the molecular mechanism of the binding. Moreover, it is conceivable that this binding actually represents an in vitro model for neurodegeneration.

Selective masking of M1-receptors in calf retina membranes by the venom of the marine snail Conus tessulatus

The non discriminatory antagonist [3H]QNB labels M1- and M2-muscarinic receptors in calf retina membranes. Crude venom from the marine gastropod Conus tessulatus produces a partial decrease in [3H]QNB binding. The total number of sites (560 +/- 13 fmol/mg protein in control experiments) decreases to 370 +/- 10 fmol/mg protein whereas the affinity of the radioligand is unaffected (KD = 0.42 +/- 0.01 nM and 0.46 +/- 0.02 nM, respectively). This process is venom concentration-dependent, quasi-irreversible, and calcium-dependent. Proteolytic activity can not be detected. The partial effect of the venom is related to preferential masking of the M1-receptors. Competition curves of the M1-selective antagonist pirenzepine are shallow in control experiments: 45% of the receptors are of the M1-type (Ki = 45 +/- 6 nM) while the remaining are of the M2-type (Ki = 1.0 +/- 0.2 microM). In venom-treated membranes, only a low affinity site (M2-receptors, Ki = 1.5 +/- 0.4 microM) is detected by pirenzepine competition binding. Saturation binding experiments reveal that the venom causes a substantial decrease in the number of high affinity sites for [3H]pirenzepine without affecting its KD (23 +/- 4 nM and 20 +/- 6 nM in control- and venom-treated membranes respectively). The venom produces a leftward shift of the carbachol/[3H]QNB competition binding curve, but the ability of 0.1 mM GTP to confer a rightward shift of the competition curve is not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 2A adrenoceptors and non-adrenergic idazoxan binding sites in calf brain and retina are distinct from those in human brain

alpha 2 Adrenoceptors in membrane preparations of human and calf frontal cortex and of calf retina can be labelled by the antagonists [3H]idazoxan, [3H]rauwolscine and [3H]RX 821002. Present and previous data indicate that [3H]idazoxan possesses the highest affinity for the alpha 2 adrenoceptors in the calf tissues, whereas [3H]rauwolscine displays the highest affinity for those in the human frontal cortex. Competition binding experiments with adrenergic and serotonergic drugs further support the notion that the alpha 2 adrenoceptors in calf frontal cortex and retina are similar, but distinct from the receptors in human frontal cortex. The alpha 2 adrenoceptors in the three investigated tissues display low affinity for the antagonist prazosin, which suggests that they all belong to the alpha 2A subclass. The competition binding curves of the alpha 2A adrenoceptor subtype-selective agonist oxymetazoline are shallow, but undergo a rightward shift and steepening in the presence of GTP. The shallow curves can therefore be attributed to the coupling of the alpha 2 adrenoceptors to G proteins. The different binding characteristics of the alpha 2A adrenoceptors from the investigated human and bovine tissues are likely to reflect species-related differences in protein structure. [3H]Idazoxan binds also to non-adrenergic sites in membrane preparations from the three tissues. However, the affinity of [3H]idazoxan for these sites in calf cortex and retina is appreciably lower than for those in human cortex. The species-related differences of the non-adrenergic idazoxan binding sites may be due to differences in protein structure or even to differences in gene-product.

Isoproterenol and selective agonists stimulate similar atypical beta-adrenoceptors in rat adipocytes

We have demonstrated previously that (-)isoproterenol triggers lipolysis in rat epididymal fat cells by stimulating both classical (beta 1, beta 2) and atypical beta-adrenoceptors. The contribution of the classical beta-adrenoceptors can be blocked by addition of 3 nM CGP12177(di-4-3[(1,1-dimethylethyl)amino]-(2-hydroxylpropoxy )1,3-dihydro-2H-benzimidazol-2-one hydrochloride). At higher concentrations, CGP12177 triggers lipolysis also, but by stimulating atypical beta-adrenoceptors only. To find out whether (-)isoproterenol and CGP12177 stimulate similar atypical beta-adrenoceptors, we compared their interaction with recognised beta 3-adrenoceptor antagonists: CGP20712 (1-[2-((3-carbamoyl-4-hydroxy)phenoxy)ethylamino]-3-[4-(1-methyl- 4-trifluoromethyl-2-imidazolyl)phenoxy]-propan-2-ol) (beta 1-selective), ICI118551 [erythro-1-(7-methylindan-4-yloxy)-3- (isopropylamine)-butan-2-ol] (beta 2-selective) and the stereoisomers as well as the racemic mixture of propranolol (non-beta 1/beta 2-subtype selective) and of metoprolol (beta 1-selective). There was a highly significant relationship (r = 0.93) between the potencies of these antagonists for inhibiting the lipolytic response to (-)isoproterenol (in the absence of classical beta-adrenoceptor stimulation) and CGP12177. In both cases, propranolol and metoprolol showed also the same degree of stereoselectivity. These findings suggest that (-)isoproterenol and CGP12177 stimulate the same type and/or form of atypical beta-adrenoceptors in rat epididymal adipocytes.

Serotonin binding proteins in bovine retina: binding of serotonin and catecholamines

Serotonin binding proteins (SBP) are present in the soluble fraction of bovine retina homogenates. These proteins can be precipitated with 30% ammonium sulphate and their binding and physicochemical characteristics are very similar to those of SBP in bovine and rat brain. Binding of [3H]serotonin to bovine retina SBP requires Fe2+ but not Fe3+. In the presence of an optimal concentration of Fe2+ (0.1 mM), these proteins behave as a single class of non-cooperative sites for [3H]serotonin (Bmax = 242 +/- 10 pmol/mg protein, KD = 0.22 +/- 0.44 microM). Competition binding studies reveal that serotonin analogs possessing an hydroxyl group on the indole ring and catecholamine analogs possessing an intact catechol moiety are potent competitors (K1 from 0.12 to 0.3 microM). In both cases, the affinity is strongly decreased if aromatic hydroxyl groups are methoxylated. Catecholamine SBP interactions can also be demonstrated directly by binding experiments with [3H]dopamine. Binding of this catecholamine is greatly enhanced by Fe2+, to a lesser extent by Cu2+ and Mn2+, but not by Fe3+. The Fe(2+)-dependent binding component is saturable (Bmax = 505 +/- 30 pmol/mg protein. KD = 0.34 +/- 0.04 microM). The SBP from bovine retina show the same physicochemical properties as SBP from bovine and rat brain: they elute immediately after the void volume on a Sephacryl S100 HR (1.6 x 140 cm) gel filtration column (reflecting aggregation) and they migrate with apparent molecular weights of respectively 43 kDa and 57 kDa on native polyacrylamide gel electrophoresis. The serotonin-storing role of SBP in serotonergic neurones has already been well documented.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple beta adrenergic receptor subclasses mediate the l-isoproterenol-induced lipolytic response in rat adipocytes

l-Isoproterenol has been proposed to stimulate lipolysis in rat epididymal adipocytes via atypical beta adrenergic receptors, whereas radioligand binding studies only revealed the presence of beta 1 adrenergic receptors on adipocyte membranes. We have made use of the unique properties of CGP12177 to evidence that both the beta 1 and the atypical beta adrenergic receptor subtypes are mediating the lipolytic response of rat epididymal adipocytes to l-isoproterenol. CGP12177, an antagonist with high affinity for beta 1 receptors, triggers lipolysis by specifically stimulating the atypical receptors. For this response, CGP12177 displays low potency (EC50 = 68 nM), but high intrinsic activity (94% relative to l-isoproterenol). At low concentrations (3 nM), CGP12177 inhibits the lipolytic response to 10 nM l-isoproterenol by 43%, indicating that at least this fraction of the response is beta 1 receptor-mediated. The response to BRL37344, which is a selective agonist for the atypical receptors, is not inhibited by CGP12177. The pA2 values of the beta adrenergic antagonists propranolol, metoprolol and atenolol were calculated from the rightward shifts that they impose on dose-response curves of both l-isoproterenol and CGP12177. With l-isoproterenol, these values (6.54, 5.83 and 5.07, respectively) are lower than those expected for beta 1 and beta 2 receptors, indicating that atypical receptors are also involved in the lipolytic response to this agonist. With CGP12177, the pA2 values of propranolol, metoprolol and atenolol are even lower (5.80, 5.03 and 4.06, respectively), and are likely to be a more accurate reflection of their affinities for the atypical receptors.

Regional Distribution of ?2A-and ?2B-Adrenoceptor Subtypes in Postmortem Human Brain

The newly available and highly selective radiolabeled antagonist [3H]RX 821002 was used to examine the distribution of alpha 2 adrenoceptors in human brain. High densities of alpha 2 adrenoceptors were found in the hippocampus, frontal cortex, thalamus, amygdala, pons, and medulla oblongata. Intermediate densities were observed in the striatum (nucleus accumbens, nucleus caudatus, and putamen), globus pallidus, and substantia nigra. The KD values for [3H]RX 821002 were similar in all regions (ranging from 2.8 to 7.5 nM). On the basis of their different affinities for prazosin and oxymetazoline, the alpha 2 adrenoceptors have been divided into alpha 2A and alpha 2B subtypes. To examine the alpha 2A/alpha 2B-adrenoceptor ratio in the different brain regions, we performed oxymetazoline and prazosin/[3H]RX 821002 competition binding experiments. In frontal cortex membranes, the competition curves with prazosin were steep, indicating a single class of binding sites, whereas the competition curves with oxymetazoline were shallow and fitted by computer best to a two-site model. However, in the presence of GTP, the high-affinity sites for oxymetazoline were partially converted into low-affinity sites, indicating that this agonist interacts with high- and low-affinity states of the alpha 2 adrenoceptors. This implies that oxymetazoline is not very suitable for discriminating the alpha 2A- and alpha 2B-receptor subtypes in radioligand binding studies. Therefore, prazosin/[3H]RX 821002 competition binding experiments were used to investigate the distribution of the alpha 2-adrenoceptor subtypes in human brain. The alpha 2A-receptor subtype was detected in all brain regions examined. In contrast, alpha 2B receptors were only observed in striatum and globus pallidus.

Serotonin-binding proteins in the bovine cerebral cortex: interaction with serotonin and catecholamines

The soluble serotonin-binding proteins (SBP) present in bovine frontal cortex are very similar to those reported in rat brain. Binding of [3H]serotonin to SBP, present in ammonium sulphate-precipitated proteins from bovine cortex, requires Fe2+ but not Fe3+. In the presence of an optimal concentration of Fe2+ (0.1 mM), bovine SBP behave as a single class of non-cooperative sites for [3H]serotonin binding (Bmax = 120 +/- 12 pmol/mg protein, KD = 0.12 +/- 0.04 microM, n = 3). Binding of [3H]serotonin is decreased by nucleotides and by reagents which modify sulfhydryl groups and reduce disulfide bonds and by metal ion chelators. Serotonin analogs possessing an hydroxyl group on the indole ring and catecholamine analogs possessing an intact catechol moiety are effective competitors (Ki from 0.1 to 0.3 microM). In both cases, the aliphatic amino group does not contribute to the binding, but the affinity is strongly decreased if aromatic hydroxyl groups are methoxylated. Catecholamine-SBP interactions can also be demonstrated directly by binding experiments. Binding of [3H]dopamine is greatly enhanced by Fe2+, Cu2+ and Mn2+, but not by Fe3+. The Fe(2+)-dependent binding component of [3H]dopamine is saturable (Bmax = 279 +/- 64 pmol/mg protein, KD = 0.19 +/- 0.02 microM, n = 3), and possesses the same physicochemical properties as SBP: it elutes immediately after the void volume on a Sephacryl S100 HR (1.6 x 140 cm) gel filtration column (reflecting aggregation) and it migrates with an apparent molecular weight of 57-58 kDa on native polyacrylamide gel electrophoresis. Whereas the serotonin-storing role of SBP in serotonergic neurons has already been well documented, the present data advocate that these proteins may also possess catecholamine-storing properties.

Inhibition of evoked neurotransmitter release from rat hippocampus by a polypeptide toxin isolated from the marine snail Conus distans

The active fraction, isolated and partially purified from the crude venom of the marine snail Conus distans, with a molecular mass of about 25 kDa, inhibits neurotransmitter release in rat hippocampus. This toxin (distans Toxin) inhibits the electrically evoked tritium labelled noradrenaline release from rat hippocampal slices in a dose and time dependent manner. The neurotransmitter release is mainly regulated by N-type of voltage sensitive Ca(2+)-channels. The distans toxin behaves as a partial antagonist of calcium in the buffer, possibly by competing with calcium for this type of voltage sensitive Ca(2+)-channels.

Autoradiographic distribution of alpha 2 adrenoceptors, NAIBS, and 5-HT1A receptors in human brain using [3H]idazoxan and [3H]rauwolscine

The regional distribution of [3H]idazoxan and [3H]rauwolscine was studied autoradiographically in human brain. [3H]Idazoxan binds with high affinity to alpha 2 adrenoceptors as well as to non-adrenergic sites (NAIBS). [3H]Rauwolscine, besides binding to alpha 2 adrenoceptors, also binds to 5-HT1A receptors. Both radioligands labelled the same population of alpha 2 adrenoceptors, defined as the epinephrine-displaceable binding component. The highest densities of alpha 2 adrenoceptors occur in the leptomeninges, cerebral cortex and claustrum; lower densities were visualised in the basal ganglia, thalamus, pons, substantia nigra, cerebellum and medulla oblongata; no alpha 2 adrenoceptors were detected in amygdala and nucleus ruber. NAIBS were present in all the examined brain areas, with the highest densities found in the basal ganglia and substantia nigra. The finding that certain brain regions, such as the amygdala, contained NAIBS but no detectable alpha 2 adrenoceptors, suggests that the binding sites are independent from each other. The regional distribution of 5-HT1A receptors labelled by [3H]rauwolscine is in agreement with previous studies using [3H]8-OH-DPAT.

Receptors for corticotropin-releasing hormone in human pituitary: binding characteristics and autoradiographic localization to immunocytochemically defined proopiomelanocortin cells

Using autoradiography combined with immunocytochemistry, we demonstrated that the target cells of CRH in the human pituitary were proopiomelanocortin cells. Scatchard analysis of [125I]Tyr0-oCRH saturation binding revealed the presence of one class of saturable, high affinity sites on pituitary tissue homogenate. The equilibrium dissociation constant (Kd) for [125I]Tyr0-oCRH ranged from 1.1-1.6 nM, and the receptor density was between 200-350 fmol/mg protein. Fixation of cryostat sections with 4% paraformaldehyde before tracer incubation improved both tissue preservation and localization of the CRH receptor at the cellular level. Additional postfixation with 1% glutaraldehyde inhibited tracer diffusion during subsequent immunocytochemistry and autoradiography. [125I]Tyr0-oCRH was found in cytoplasmic inclusions or at the cell periphery of ACTH/beta-endorphin cells in the anterior pituitary. Single cells of the posterior pituitary were also CRH receptor positive. Cells staining for PRL or GH were CRH receptor negative. We conclude that CRH binds only to high affinity receptors on ACTH/beta-endorphin cells in the human pituitary.

[3H]rauwolscine behaves as an agonist for the 5-HT1A receptors in human frontal cortex membranes

The alpha 2 adrenergic antagonist [3H]rauwolscine binds with comparable nanomolar affinity to alpha 2 adrenoceptors and the nonadrenergic 5-HT1A receptors sites in human frontal cortex membranes. Addition of 0.5 mM GTP into the incubation medium produces a significant decrease in the amount of [3H]rauwolscine binding sites (Bmax = 230 +/- 16 and 115 +/- 11 fmol/mg protein in the absence and presence of GTP, respectively). The affinity for [3H]rauwolscine remains unchanged (i.e. KD = 40 +/- 0.9 nM and 4.1 +/- 1 nM). This effect of GTP can be attributed to decreased binding of the radioligand to the 5-HT1A receptors. GTP decreases binding of [3H]rauwolscine to nearly the same level as the one corresponding to the alpha 2 adrenoceptors in membranes from both the human frontal cortex and hippocampus. The venom of the marine cone snail, Conus tessulatus, preferentially inhibits [3H]rauwolscine binding to 5-HT1A receptors as compared with the alpha 2 adrenoceptors. Following complete masking of the 5-HT1A receptors by this venom. GTP no longer affects the saturation binding characteristics of [3H]rauwolscine for the remaining alpha 2 adrenoceptors. Nucleotides decrease the binding of [3H]rauwolscine to the 5-HT1A receptors with an order of potencies (i.e. GTP gamma S greater than GPP(NH)P much greater than GDP greater than GTP much greater than ATP) that is typical for nucleotide-mediated receptor-G protein dissociation. This suggests that [3H]rauwolscine is a 5-HT1A receptor agonist and this conclusion is compatible with earlier functional studies, indicating that rauwolscine (as well as yohimbine) has agonistic properties at the level of 5-HT autoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

D1 and D2 dopamine receptors in human substantia nigra: localization and the effect of aging

D1 and D2 receptor densities in human substantia nigra were examined by use of the specific binding of, respectively, [3H]SCH 23390 [R(+)-7-chloro-8-hydroxy-3-[3H]methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine] and [3H]spiperone. A unilateral loss of striato- and pallidonigral pathways by an infarction (n = 4) had no effect on the ipsilateral nigral D2 receptors, but reduced the ipsilateral nigral D1 receptors by 48-60% compared with the intact side. These data suggest that a substantial fraction of D1 receptors in human substantia nigra is located on terminals of striato- and/or pallidonigral neurons, whereas D2 receptors are confined to intrinsic nigral cells. We also examined the effect of aging on the D1 and D2 receptors in substantia nigra obtained from 25 postmortem human brains (age range 19-88 years). The densities of both receptor types were not affected by the aging process. Since nigrostriatal dopaminergic neurons degenerate with aging, these results suggest either that the nigral D2 receptors are up-regulated in response to a progressive depletion of dopamine in the substantia nigra or that, in contrast to the rat, they are not located on dopaminergic neurons.